Ultrafine bubble generating method, ultrafine bubble generating apparatus, and ultrafine bubble-containing liquid

ABSTRACT

An ultrafine bubble generating apparatus and an ultrafine bubble generating method capable of efficiently generating an ultrafine bubble-containing liquid with high purity are provided. In order to this, a heating element provided in a liquid is caused to generate heat, and film boiling is made on an interface between the liquid and the heating element. A film boiling bubble is generated by the film boiling, and ultrafine bubbles are thus generated near the film boiling bubble.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an ultrafine bubble generating methodand an ultrafine bubble generating apparatus for generating ultrafinebubbles smaller than 1.0 μm in diameter, and an ultrafinebubble-containing liquid.

Description of the Related Art

Recently, there have been developed techniques for applying the featuresof fine bubbles such as microbubbles in micrometer-size in diameter andnanobubbles in nanometer-size in diameter. Especially, the utility ofultrafine bubbles (hereinafter also referred to as “UFBs”) smaller than1.0 μm in diameter have been confirmed in various fields.

Japanese Patent No. 6118544 discloses a fine air bubble generatingapparatus that generates fine bubbles by ejecting from a depressurizingnozzle a pressurized liquid in which a gas is pressurized and dissolved.Japanese Patent No. 4456176 discloses an apparatus that generates finebubbles by repeating separating and converging of flows of a gas-mixedliquid with a mixing unit.

Both the apparatuses described in Japanese Patent Nos. 6118544 and4456176 generate not only the UFBs of nanometer-size in diameter butalso relatively a large number of milli-bubbles of millimeter-size indiameter and microbubbles of micrometer-size in diameter. However,because the milli-bubbles and the microbubbles are affected by thebuoyancy, the bubbles are likely to gradually rise to the liquid surfaceand disappear during long-time storage.

On the other hand, the UFBs of nanometer-size in diameter are suitablefor long-time storage since they are less likely to be affected by thebuoyancy and float in the liquid with Brownian motion. However, when theUFBs are generated with the milli-bubbles and the microbubbles or thegas-liquid interface energy of the UFBs is small, the UFBs are affectedby the disappearance of the milli-bubbles and the microbubbles anddecreased over time.

SUMMARY OF THE INVENTION

That is, in order to obtain a UFB-containing liquid in which theconcentration reduction of the UFBs can be suppressed even duringlong-time storage, it is required to generate highly pure and highlyconcentrated UFBs with large gas-liquid interface energy when generatinga UFB-containing liquid.

The present invention is made to solve the above-described problems.Therefore, an object of the present invention is to provide an ultrafinebubble generating apparatus and an ultrafine bubble generating methodcapable of efficiently generating a UFB-containing liquid with highpurity.

In a first aspect of the present invention, there is provided anultrafine bubble generating method for generating ultrafine bubbles bycausing a heating element provided in a liquid to generate heat, makingfilm boiling on an interface between the liquid and the heating element,and generating a film boiling bubble.

In a second aspect of the present invention, there is provided anultrafine bubble generating method for generating ultrafine bubblescontaining a predetermined gas component by causing a heating elementprovided in a liquid in which the gas component is dissolved in advanceto generate heat, making film boiling on an interface between the liquidand the heating element, and generating a film boiling bubble.

In a third aspect of the present invention, there is provided anultrafine bubble generating method, comprising: dissolving apredetermined gas component into a liquid; and generating ultrafinebubbles containing the predetermined gas component by causing a heatingelement provided in the liquid in which the gas component is dissolvedin the dissolving step to generate heat, making film boiling on aninterface between the liquid and the heating element, and generating afilm boiling bubble.

In a fourth aspect of the present invention, there is provided anultrafine bubble generating method, comprising: generating ultrafinebubbles containing a predetermined gas component by causing a heatingelement provided in a liquid in which the gas component is dissolved inadvance to generate heat, making film boiling on an interface betweenthe liquid and the heating element, and generating a film boilingbubble; and collecting the liquid containing the ultrafine bubblesgenerated in the generating step.

In a fifth aspect of the present invention, there is provided anultrafine bubble generating apparatus, comprising: a heating element;and a driving unit configured to drive the heating element, wherein theultrafine bubble generating apparatus generates ultrafine bubbles bycausing the heating element provided in a liquid with the driving unitto generate heat, making film boiling on an interface between the liquidand the heating element, and generating a film boiling bubble.

In a sixth aspect of the present invention, there is provided anultrafine bubble-containing liquid that contains ultrafine bubblesgenerated by causing a heating element provided in a liquid to generateheat, making film boiling on an interface between the liquid and theheating element, and generating a film boiling bubble.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of a UFB generatingapparatus;

FIG. 2 is a schematic configuration diagram of a pre-processing unit;

FIGS. 3A and 3B are a schematic configuration diagram of a dissolvingunit and a diagram for describing the dissolving states in a liquid;

FIG. 4 is a schematic configuration diagram of a T-UFB generating unit;

FIGS. 5A and 5B are diagrams for describing details of a heatingelement;

FIGS. 6A and 6B are diagrams for describing the states of film boilingon the heating element;

FIGS. 7A to 7D are diagrams illustrating the states of generation ofUFBs caused by expansion of a film boiling bubble;

FIGS. 8A to 8C are diagrams illustrating the states of generation ofUFBs caused by shrinkage of the film boiling bubble;

FIGS. 9A to 9C are diagrams illustrating the states of generation ofUFBs caused by reheating of the liquid;

FIGS. 10A and 10B are diagrams illustrating the states of generation ofUFBs caused by shock waves made by disappearance of the bubble generatedby the film boiling;

FIGS. 11A to 11C are diagrams illustrating a configuration example of apost-processing unit;

FIGS. 12A to 12C are plane diagrams of a heating element of Embodiment1;

FIGS. 13A and 13B are plane diagrams of a heating element of Embodiment2;

FIGS. 14A to 14C are plane diagrams illustrating the states of a filmboiling bubble and a flow passage of Embodiment 3;

FIGS. 15A and 15B are plane diagrams illustrating the states of a filmboiling bubble and a flow passage of Embodiment 4; and

FIGS. 16A to 16C are plane diagrams illustrating another example of thestates of the film boiling bubble and the flow passages of Embodiment 4.

DESCRIPTION OF THE EMBODIMENTS

<<Configuration of UFB Generating Apparatus>>

FIG. 1 is a diagram illustrating an example of a UFB generatingapparatus applicable to the present invention. A UFB generatingapparatus 1 of this embodiment includes a pre-processing unit 100,dissolving unit 200, a T-UFB generating unit 300, a post-processing unit400, and a collecting unit 500. Each unit performs unique processing ona liquid W such as tap water supplied to the pre-processing unit 100 inthe above order, and the thus-processed liquid W is collected as aT-UFB-containing liquid by the collecting unit 500. Functions andconfigurations of the units are described below. Although details aredescribed later, UFBs generated by utilizing the film boiling caused byrapid heating are referred to as thermal-ultrafine bubbles (T-UFBs) inthis specification.

FIG. 2 is a schematic configuration diagram of the pre-processing unit100. The pre-processing unit 100 of this embodiment performs a degassingtreatment on the supplied liquid W. The pre-processing unit 100 mainlyincludes a degassing container 101, a shower head 102, a depressurizingpump 103, a liquid introduction passage 104, a liquid circulationpassage 105, and a liquid discharge passage 106. For example, the liquidW such as tap water is supplied to the degassing container 101 from theliquid introduction passage 104 through a valve 109. In this process,the shower head 102 provided in the degassing container 101 sprays amist of the liquid W in the degassing container 101. The shower head 102is for prompting the gasification of the liquid W; however, acentrifugal and the like may be used instead as the mechanism forproducing the gasification prompt effect.

When a certain amount of the liquid W is reserved in the degassingcontainer 101 and then the depressurizing pump 103 is activated with allthe valves closed, already-gasified gas components are discharged, andgasification and discharge of gas components dissolved in the liquid Ware also prompted. In this process, the internal pressure of thedegassing container 101 may be depressurized to around several hundredsto thousands of Pa (1.0 Torr to 10.0 Torr) while checking a manometer108. The gases to be removed by the pre-processing unit 100 includesnitrogen, oxygen, argon, carbon dioxide, and so on, for example.

The above-described degassing processing can be repeatedly performed onthe same liquid W by utilizing the liquid circulation passage 105.Specifically, the shower head 102 is operated with the valve 109 of theliquid introduction passage 104 and a valve 110 of the liquid dischargepassage 106 closed and a valve 107 of the liquid circulation passage 105opened. This allows the liquid W reserved in the degassing container 101and degassed once to be resprayed in the degassing container 101 fromthe shower head 102. In addition, with the depressurizing pump 103operated, the gasification processing by the shower head 102 and thedegassing processing by the depressurizing pump 103 are repeatedlyperformed on the same liquid W. Every time the above processingutilizing the liquid circulation passage 105 is performed repeatedly, itis possible to decrease the gas components contained in the liquid W instages. Once the liquid W degassed to a desired purity is obtained, theliquid W is transferred to the dissolving unit 200 through the liquiddischarge passage 106 with the valve 110 opened.

FIG. 2 illustrates the degassing unit 100 that depressurizes the gaspart to gasify the solute; however, the method of degassing the solutionis not limited thereto. For example, a heating and boiling method forboiling the liquid W to gasify the solute may be employed, or a filmdegassing method for increasing the interface between the liquid and thegas using hollow fibers. A SEPAREL series (produced by DIC corporation)is commercially supplied as the degassing module using the hollowfibers. The SEPAREL series uses poly(4-methylpentene-1) (PMP) for theraw material of the hollow fibers and is used for removing air bubblesfrom ink and the like mainly supplied for a piezo head. In addition, twoor more of an evacuating method, the heating and boiling method, and thefilm degassing method may be used together.

FIGS. 3A and 3B are a schematic configuration diagram of the dissolvingunit 200 and a diagram for describing the dissolving states in theliquid. The dissolving unit 200 is a unit for dissolving a desired gasinto the liquid W supplied from the pre-processing unit 100. Thedissolving unit 200 of this embodiment mainly includes a dissolvingcontainer 201, a rotation shaft 203 provided with a rotation plate 202,a liquid introduction passage 204, a gas introduction passage 205, aliquid discharge passage 206, and a pressurizing pump 207.

The liquid W supplied from the pre-processing unit 100 is supplied andreserved into the dissolving container 201 through the liquidintroduction passage 204. Meanwhile, a gas G is supplied to thedissolving container 201 through the gas introduction passage 205.

Once predetermined amounts of the liquid W and the gas G are reserved inthe dissolving container 201, the pressurizing pump 207 is activated toincrease the internal pressure of the dissolving container 201 to about0.5 MPa. A safety valve 208 is arranged between the pressurizing pump207 and the dissolving container 201. With the rotation plate 202 in theliquid rotated via the rotation shaft 203, the gas G supplied to thedissolving container 201 is transformed into air bubbles, and thecontact area between the gas G and the liquid W is increased to promptthe dissolution into the liquid W. This operation is continued until thesolubility of the gas G reaches almost the maximum saturationsolubility. In this case, a unit for decreasing the temperature of theliquid may be provided to dissolve the gas as much as possible. When thegas is with low solubility, it is also possible to increase the internalpressure of the dissolving container 201 to 0.5 MPa or higher. In thiscase, the material and the like of the container need to be the optimumfor safety sake.

Once the liquid W in which the components of the gas G are dissolved ata desired concentration is obtained, the liquid W is discharged throughthe liquid discharge passage 206 and supplied to the T-UFB generatingunit 300. In this process, a back-pressure valve 209 adjusts the flowpressure of the liquid W to prevent excessive increase of the pressureduring the supplying.

FIG. 3B is a diagram schematically illustrating the dissolving states ofthe gas G put in the dissolving container 201. An air bubble 2containing the components of the gas G put in the liquid W is dissolvedfrom a portion in contact with the liquid W. The air bubble 2 thusshrinks gradually, and a gas-dissolved liquid 3 then appears around theair bubble 2. Since the air bubble 2 is affected by the buoyancy, theair bubble 2 may be moved to a position away from the center of thegas-dissolved liquid 3 or be separated out from the gas-dissolved liquid3 to become a residual air bubble 4. Specifically, in the liquid W to besupplied to the T-UFB generating unit 300 through the liquid dischargepassage 206, there is a mix of the air bubbles 2 surrounded by thegas-dissolved liquids 3 and the air bubbles 2 and the gas-dissolvedliquids 3 separated from each other.

The gas-dissolved liquid 3 in the drawings means “a region of the liquidW in which the dissolution concentration of the gas G mixed therein isrelatively high.” In the gas components actually dissolved in the liquidW, the concentration of the gas components in the gas-dissolved liquid 3is the highest at a portion surrounding the air bubble 2. In a casewhere the gas-dissolved liquid 3 is separated from the air bubble 2 theconcentration of the gas components of the gas-dissolved liquid 3 is thehighest at the center of the region, and the concentration iscontinuously decreased as away from the center. That is, although theregion of the gas-dissolved liquid 3 is surrounded by a broken line inFIG. 3 for the sake of explanation, such a clear boundary does notactually exist. In addition, in the present invention, a gas that cannotbe dissolved completely may be accepted to exist in the form of an airbubble in the liquid.

FIG. 4 is a schematic configuration diagram of the T-UFB generating unit300. The T-UFB generating unit 300 mainly includes a chamber 301, aliquid introduction passage 302, and a liquid discharge passage 303. Theflow from the liquid introduction passage 302 to the liquid dischargepassage 303 through the chamber 301 is formed by a not-illustrated flowpump. Various pumps including a diaphragm pump, a gear pump, and a screwpump may be employed as the flow pump. In in the liquid W introducedfrom the liquid introduction passage 302, the gas-dissolved liquid 3 ofthe gas G put by the dissolving unit 200 is mixed.

An element substrate 12 provided with a heating element 10 is arrangedon a bottom section of the chamber 301. With a predetermined voltagepulse applied to the heating element 10, a bubble 13 generated by thefilm boiling (hereinafter, also referred to as a film boiling bubble 13)is generated in a region in contact with the heating element 10. Then,an ultrafine bubble (UFB) 11 containing the gas G is generated caused byexpansion and shrinkage of the film boiling bubble 13. As a result, aUFB-containing liquid W containing many UFBs 11 is discharged from theliquid discharge passage 303.

FIGS. 5A and 5B are diagrams for illustrating a detailed configurationof the heating element 10. FIG. 5A illustrates a closeup view of theheating element 10, and FIG. 5B illustrates a cross-sectional view of awider region of the element substrate 12 including the heating element10.

As illustrated in FIG. 5A, in the element substrate 12 of thisembodiment, a thermal oxide film 305 as a heat-accumulating layer and aninterlaminar film 306 also served as a heat-accumulating layer arelaminated on a surface of a silicon substrate 304. An SiO₂ film or anSiN film may be used as the interlaminar film 306. A resistive layer 307is formed on a surface of the interlaminar film 306, and a wiring 308 ispartially formed on a surface of the resistive layer 307. An Al-alloywiring of Al, Al—Si, Al—Cu, or the like may be used as the wiring 308. Aprotective layer 309 made of an SiO₂ film or an Si₃N₄ film is formed onsurfaces of the wiring 308, the resistive layer 307, and theinterlaminar film 306.

A cavitation-resistant film 310 for protecting the protective layer 309from chemical and physical impacts due to the heat evolved by theresistive layer 307 is formed on a portion and around the portion on thesurface of the protective layer 309, the portion corresponding to aheat-acting portion 311 that eventually becomes the heating element 10.A region on the surface of the resistive layer 307 in which the wiring308 is not formed is the heat-acting portion 311 in which the resistivelayer 307 evolves heat. The heating portion of the resistive layer 307on which the wiring 308 is not formed functions as the heating element(heater) 10. As described above, the layers in the element substrate 12are sequentially formed on the surface of the silicon substrate 304 by asemiconductor production technique, and the heat-acting portion 311 isthus provided on the silicon substrate 304.

The configuration illustrated in the drawings is an example, and variousother configurations are applicable. For example, a configuration inwhich the laminating order of the resistive layer 307 and the wiring 308is opposite, and a configuration in which an electrode is connected to alower surface of the resistive layer 307 (so-called a plug electrodeconfiguration) are applicable. In other words, as described later, anyconfiguration may be applied as long as the configuration allows theheat-acting portion 311 to heat the liquid for generating the filmboiling in the liquid.

FIG. 5B is an example of a cross-sectional view of a region including acircuit connected to the wiring 308 in the element substrate 12. AnN-type well region 322 and a P-type well region 323 are partiallyprovided in a top layer of the silicon substrate 304, which is a P-typeconductor. AP-MOS 320 is formed in the N-type well region 322 and anN-MOS 321 is formed in the P-type well region 323 by introduction anddiffusion of impurities by the ion implantation and the like in thegeneral MOS process.

The P-MOS 320 includes a source region 325 and a drain region 326 formedby partial introduction of N-type or P-type impurities in a top layer ofthe N-type well region 322, a gate wiring 335, and so on. The gatewiring 335 is deposited on a part of a top surface of the N-type wellregion 322 excluding the source region 325 and the drain region 326,with a gate insulation film 328 of several hundreds of A in thicknessinterposed between the gate wiring 335 and the top surface of the N-typewell region 322.

The N-MOS 321 includes the source region 325 and the drain region 326formed by partial introduction of N-type or P-type impurities in a toplayer of the P-type well region 323, the gate wiring 335, and so on. Thegate wiring 335 is deposited on a part of a top surface of the P-typewell region 323 excluding the source region 325 and the drain region326, with the gate insulation film 328 of several hundreds of A inthickness interposed between the gate wiring 335 and the top surface ofthe P-type well region 323. The gate wiring 335 is made of polysiliconof 3000 Å to 5000 Å in thickness deposited by the CVD method. A C-MOSlogic is constructed with the P-MOS 320 and the N-MOS 321.

In the P-type well region 323, an N-MOS transistor 330 for driving anelectrothermal conversion element (heating resistance element) is formedon a portion different from the portion including the N-MOS 321. TheN-MOS transistor 330 includes a source region 332 and a drain region 331partially provided in the top layer of the P-type well region 323 by thesteps of introduction and diffusion of impurities, a gate wiring 333,and so on. The gate wiring 333 is deposited on a part of the top surfaceof the P-type well region 323 excluding the source region 332 and thedrain region 331, with the gate insulation film 328 interposed betweenthe gate wiring 333 and the top surface of the P-type well region 323.

In this example, the N-MOS transistor 330 is used as the transistor fordriving the electrothermal conversion element. However, the transistorfor driving is not limited to the N-MOS transistor 330, and anytransistor may be used as long as the transistor has a capability ofdriving multiple electrothermal conversion elements individually and canimplement the above-described fine configuration. Although theelectrothermal conversion element and the transistor for driving theelectrothermal conversion element are formed on the same substrate inthis example, those may be formed on different substrates separately.

An oxide film separation region 324 is formed by field oxidation of 5000Å to 10000 Å in thickness between the elements, such as between theP-MOS 320 and the N-MOS 321 and between the N-MOS 321 and the N-MOStransistor 330. The oxide film separation region 324 separates theelements. A portion of the oxide film separation region 324corresponding to the heat-acting portion 311 functions as aheat-accumulating layer 334, which is the first layer on the siliconsubstrate 304.

An interlayer insulation film 336 including a PSG film, a BPSG film, orthe like of about 7000 Å in thickness is formed by the CVD method oneach surface of the elements such as the P-MOS 320, the N-MOS 321, andthe N-MOS transistor 330. After the interlayer insulation film 336 ismade flat by heat treatment, an Al electrode 337 as a first wiring layeris formed in a contact hole penetrating through the interlayerinsulation film 336 and the gate insulation film 328. On surfaces of theinterlayer insulation film 336 and the Al electrode 337, an interlayerinsulation film 338 including an SiO₂ film of 10000 Å to 15000 Å inthickness is formed by a plasma CVD method. On the surface of theinterlayer insulation film 338, a resistive layer 307 including a TaSiNfilm of about 500 Å in thickness is formed by a co-sputter method onportions corresponding to the heat-acting portion 311 and the N-MOStransistor 330. The resistive layer 307 is electrically connected withthe Al electrode 337 near the drain region 331 via a through-hole formedin the interlayer insulation film 338. On the surface of the resistivelayer 307, the wiring 308 of Al as a second wiring layer for a wiring toeach electrothermal conversion element is formed. The protective layer309 on the surfaces of the wiring 308, the resistive layer 307, and theinterlayer insulation film 338 includes an SiN film of 3000 Å inthickness formed by the plasma CVD method. The cavitation-resistant film310 deposited on the surface of the protective layer 309 includes a thinfilm of about 2000 Å in thickness, which is at least one metal selectedfrom the group consisting of Ta, Fe, Ni, Cr, Ge, Ru, Zr, Ir, and thelike. Various materials other than the above-described TaSiN such asTaN0.8, CrSiN, TaAl, WSiN, and the like can be applied as long as thematerial can generate the film boiling in the liquid.

FIGS. 6A and 6B are diagrams illustrating the states of the film boilingwhen a predetermined voltage pulse is applied to the heating element 10.In this case, the case of generating the film boiling under atmosphericpressure is described. In FIG. 6A, the horizontal axis represents time.The vertical axis in the lower graph represents a voltage applied to theheating element 10, and the vertical axis in the upper graph representsthe volume and the internal pressure of the film boiling bubble 13generated by the film boiling. On the other hand, FIG. 6B illustratesthe states of the film boiling bubble 13 in association with timings 1to 3 shown in FIG. 6A. Each of the states is described below inchronological order. The UFBs 11 generated by the film boiling asdescribed later are mainly generated near a surface of the film boilingbubble 13. The states illustrated in FIG. 6B are the states where theUFBs 11 generated by the generating unit 300 are resupplied to thedissolving unit 200 through the circulation route, and the liquidcontaining the UFBs 11 is resupplied to the liquid passage of thegenerating unit 300, as illustrated in FIG. 1 .

Before a voltage is applied to the heating element 10, the atmosphericpressure is substantially maintained in the chamber 301. Once a voltageis applied to the heating element 10, the film boiling is generated inthe liquid in contact with the heating element 10, and a thus-generatedair bubble (hereinafter, referred to as the film boiling bubble 13) isexpanded by a high pressure acting from inside (timing 1). A bubblingpressure in this process is expected to be around 8 to 10 MPa, which isa value close to a saturation vapor pressure of water.

The time for applying a voltage (pulse width) is around 0.5 pec to 10.0pec, and the film boiling bubble 13 is expanded by the inertia of thepressure obtained in timing 1 even after the voltage application.However, a negative pressure generated with the expansion is graduallyincreased inside the film boiling bubble 13, and the negative pressureacts in a direction to shrink the film boiling bubble 13. After a while,the volume of the film boiling bubble 13 becomes the maximum in timing 2when the inertial force and the negative pressure are balanced, andthereafter the film boiling bubble 13 shrinks rapidly by the negativepressure.

In the disappearance of the film boiling bubble 13, the film boilingbubble 13 disappears not in the entire surface of the heating element 10but in one or more extremely small regions. For this reason, on theheating element 10, further greater force than that in the bubbling intiming 1 is generated in the extremely small region in which the filmboiling bubble 13 disappears (timing 3).

The generation, expansion, shrinkage, and disappearance of the filmboiling bubble 13 as described above are repeated every time a voltagepulse is applied to the heating element 10, and new UFBs 11 aregenerated each time.

The states of generation of the UFBs 11 in each process of thegeneration, expansion, shrinkage, and disappearance of the film boilingbubble 13 are further described in detail with reference to FIGS. 7A to10B.

FIGS. 7A to 7D are diagrams schematically illustrating the states ofgeneration of the UFBs 11 caused by the generation and the expansion ofthe film boiling bubble 13. FIG. 7A illustrates the state before theapplication of a voltage pulse to the heating element 10. The liquid Win which the gas-dissolved liquids 3 are mixed flows inside the chamber301.

FIG. 7B illustrates the state where a voltage is applied to the heatingelement 10, and the film boiling bubble 13 is evenly generated in almostall over the region of the heating element 10 in contact with the liquidW. When a voltage is applied, the surface temperature of the heatingelement 10 rapidly increases at a speed of 10° C./pec. The film boilingoccurs at a time point when the temperature reaches almost 300° C., andthe film boiling bubble 13 is thus generated.

Thereafter, the surface temperature of the heating element 10 keepsincreasing to around 600 to 800° C. during the pulse application, andthe liquid around the film boiling bubble 13 is rapidly heated as well.In FIG. 7B, a region of the liquid that is around the film boilingbubble 13 and to be rapidly heated is indicated as a not-yet-bubblinghigh temperature region 14. The gas-dissolved liquid 3 within thenot-yet-bubbling high temperature region 14 exceeds the thermaldissolution limit and is vaporized to become the UFB. The thus-vaporizedair bubbles have diameters of around 10 nm to 100 nm and largegas-liquid interface energy. Thus, the air bubbles float independentlyin the liquid W without disappearing in a short time. In thisembodiment, the air bubbles generated by the thermal action from thegeneration to the expansion of the film boiling bubble 13 are calledfirst UFBs 11A.

FIG. 7C illustrates the state where the film boiling bubble 13 isexpanded. Even after the voltage pulse application to the heatingelement 10, the film boiling bubble 13 continues expansion by theinertia of the force obtained from the generation thereof, and thenot-yet-bubbling high temperature region 14 is also moved and spread bythe inertia. Specifically, in the process of the expansion of the filmboiling bubble 13, the gas-dissolved liquid 3 within thenot-yet-bubbling high temperature region 14 is vaporized as a new airbubble and becomes the first UFB 11A.

FIG. 7D illustrates the state where the film boiling bubble 13 has themaximum volume. As the film boiling bubble 13 is expanded by theinertia, the negative pressure inside the film boiling bubble 13 isgradually increased along with the expansion, and the negative pressureacts to shrink the film boiling bubble 13. At a time point when thenegative pressure and the inertial force are balanced, the volume of thefilm boiling bubble 13 becomes the maximum, and then the shrinkage isstarted.

In the shrinking stage of the film boiling bubble 13, there are UFBsgenerated by the processes illustrated in FIGS. 8A to 8C (second UFBs11B) and UFBs generated by the processes illustrated in FIGS. 9A to 9C(third UFBs 11C). It is considered that these two processes are madesimultaneously.

FIGS. 8A to 8C are diagrams illustrating the states of generation of theUFBs 11 caused by the shrinkage of the film boiling bubble 13. FIG. 8Aillustrates the state where the film boiling bubble 13 starts shrinking.Although the film boiling bubble 13 starts shrinking, the surroundingliquid W still has the inertial force in the expansion direction.Because of this, the inertial force acting in the direction of goingaway from the heating element 10 and the force going toward the heatingelement 10 caused by the shrinkage of the film boiling bubble 13 act ina surrounding region extremely close to the film boiling bubble 13, andthe region is depressurized. The region is indicated in the drawings asa not-yet-bubbling negative pressure region 15.

The gas-dissolved liquid 3 within the not-yet-bubbling negative pressureregion 15 exceeds the pressure dissolution limit and is vaporized tobecome an air bubble. The thus-vaporized air bubbles have diameters ofabout 100 nm and thereafter float independently in the liquid W withoutdisappearing in a short time. In this embodiment, the air bubblesvaporized by the pressure action during the shrinkage of the filmboiling bubble 13 are called the second UFBs 11B.

FIG. 8B illustrates a process of the shrinkage of the film boilingbubble 13. The shrinking speed of the film boiling bubble 13 isaccelerated by the negative pressure, and the not-yet-bubbling negativepressure region 15 is also moved along with the shrinkage of the filmboiling bubble 13. Specifically, in the process of the shrinkage of thefilm boiling bubble 13, the gas-dissolved liquids 3 within a part overthe not-yet-bubbling negative pressure region 15 are precipitated oneafter another and become the second UFBs 11B.

FIG. 8C illustrates the state immediately before the disappearance ofthe film boiling bubble 13. Although the moving speed of the surroundingliquid W is also increased by the accelerated shrinkage of the filmboiling bubble 13, a pressure loss occurs due to a flow passageresistance in the chamber 301. As a result, the region occupied by thenot-yet-bubbling negative pressure region 15 is further increased, and anumber of the second UFBs 11B are generated.

FIGS. 9A to 9C are diagrams illustrating the states of generation of theUFBs by reheating of the liquid W during the shrinkage of the filmboiling bubble 13. FIG. 9A illustrates the state where the surface ofthe heating element 10 is covered with the shrinking film boiling bubble13.

FIG. 9B illustrates the state where the shrinkage of the film boilingbubble 13 has progressed, and a part of the surface of the heatingelement 10 comes in contact with the liquid W. In this state, there isheat left on the surface of the heating element 10, but the heat is nothigh enough to cause the film boiling even if the liquid W comes incontact with the surface. A region of the liquid to be heated by comingin contact with the surface of the heating element 10 is indicated inthe drawings as a not-yet-bubbling reheated region 16. Although the filmboiling is not made, the gas-dissolved liquid 3 within thenot-yet-bubbling reheated region 16 exceeds the thermal dissolutionlimit and is vaporized. In this embodiment, the air bubbles generated bythe reheating of the liquid W during the shrinkage of the film boilingbubble 13 are called the third UFBs 11C.

FIG. 9C illustrates the state where the shrinkage of the film boilingbubble 13 has further progressed. The smaller the film boiling bubble13, the greater the region of the heating element 10 in contact with theliquid W, and the third UFBs 11C are generated until the film boilingbubble 13 disappears.

FIGS. 10A and 10B are diagrams illustrating the states of generation ofthe UFBs caused by an impact from the disappearance of the film boilingbubble 13 generated by the film boiling (that is, a type of cavitation).FIG. 10A illustrates the state immediately before the disappearance ofthe film boiling bubble 13. In this state, the film boiling bubble 13shrinks rapidly by the internal negative pressure, and thenot-yet-bubbling negative pressure region 15 surrounds the film boilingbubble 13.

FIG. 10B illustrates the state immediately after the film boiling bubble13 disappears at a point P. When the film boiling bubble 13 disappears,acoustic waves ripple concentrically from the point P as a startingpoint due to the impact of the disappearance. The acoustic wave is acollective term of an elastic wave that is propagated through anythingregardless of gas, liquid, and solid. In this embodiment, compressionwaves of the liquid W, which are a high pressure surface 17A and a lowpressure surface 17B of the liquid W, are propagated alternately.

In this case, the gas-dissolved liquid 3 within the not-yet-bubblingnegative pressure region 15 is resonated by the shock waves made by thedisappearance of the film boiling bubble 13, and the gas-dissolvedliquid 3 exceeds the pressure dissolution limit and the phase transitionis made in timing when the low pressure surface 17B passes therethrough.Specifically, a number of air bubbles are vaporized in thenot-yet-bubbling negative pressure region 15 simultaneously with thedisappearance of the film boiling bubble 13. In this embodiment, the airbubbles generated by the shock waves made by the disappearance of thefilm boiling bubble 13 are called fourth UFBs 11D.

The fourth UFBs 11D generated by the shock waves made by thedisappearance of the film boiling bubble 13 suddenly appear in anextremely short time (1 μS or less) in an extremely narrow thinfilm-shaped region. The diameter is sufficiently smaller than that ofthe first to third UFBs, and the gas-liquid interface energy is higherthan that of the first to third UFBs. For this reason, it is consideredthat the fourth UFBs 11D have different characteristics from the firstto third UFBs 11A to 11C and generate different effects.

Additionally, the fourth UFBs 11D are evenly generated in many parts ofthe region of the concentric sphere in which the shock waves arepropagated, and the fourth UFBs 11D evenly exist in the chamber 301 fromthe generation thereof. Although many first to third UFBs already existin the timing of the generation of the fourth UFBs 11D, the presence ofthe first to third UFBs does not affect the generation of the fourthUFBs 11D greatly. It is also considered that the first to third UFBs donot disappear due to the generation of the fourth UFBs 11D.

As described above, it is expected that the UFBs 11 are generated in themultiple stages from the generation to the disappearance of the filmboiling bubble 13 by the heat generation of the heating element 10. Thefirst UFBs 11A, the second UFBs 11B, and the third UFBs 11C aregenerated near the surface of the film boiling bubble generated by thefilm boiling. In this case, near means a region within about 20 μm fromthe surface of the film boiling bubble. The fourth UFBs 11D aregenerated in a region through which the shock waves are propagated whenthe air bubble disappears. Although the above example illustrates thestages to the disappearance of the film boiling bubble 13, the way ofgenerating the UFBs is not limited thereto. For example, with thegenerated film boiling bubble 13 communicating with the atmospheric airbefore the bubble disappearance, the UFBs can be generated also if thefilm boiling bubble 13 does not reach the disappearance.

Next, remaining properties of the UFBs are described. The higher thetemperature of the liquid, the lower the dissolution properties of thegas components, and the lower the temperature, the higher thedissolution properties of the gas components. In other words, the phasetransition of the dissolved gas components is prompted and thegeneration of the UFBs becomes easier as the temperature of the liquidis higher. The temperature of the liquid and the solubility of the gasare in the inverse relationship, and the gas exceeding the saturationsolubility is transformed into air bubbles and appeared in the liquid asthe liquid temperature increases.

Therefore, when the temperature of the liquid rapidly increases fromnormal temperature, the dissolution properties are decreased withoutstopping, and the generation of the UFBs starts. The thermal dissolutionproperties are decreased as the temperature increases, and a number ofthe UFBs are generated.

Conversely, when the temperature of the liquid decreases from normaltemperature, the dissolution properties of the gas are increased, andthe generated UFBs are more likely to be liquefied. However, suchtemperature is sufficiently lower than normal temperature. Additionally,since the once generated UFBs have a high internal pressure and largegas-liquid interface energy even when the temperature of the liquiddecreases, it is highly unlikely that there is exerted a sufficientlyhigh pressure to break such a gas-liquid interface. In other words, theonce generated UFBs do not disappear easily as long as the liquid isstored at normal temperature and normal pressure.

In this embodiment, the first UFBs 11A described with FIGS. 7A to 7C andthe third UFBs 11C described with FIGS. 9A to 9C can be described asUFBs that are generated by utilizing such thermal dissolution propertiesof gas.

On the other hand, in the relationship between the pressure and thedissolution properties of liquid, the higher the pressure of the liquid,the higher the dissolution properties of the gas, and the lower thepressure, the lower the dissolution properties. In other words, thephase transition to the gas of the gas-dissolved liquid dissolved in theliquid is prompted and the generation of the UFBs becomes easier as thepressure of the liquid is lower. Once the pressure of the liquid becomeslower than normal pressure, the dissolution properties are decreasedinstantly, and the generation of the UFBs starts. The pressuredissolution properties are decreased as the pressure decreases, and anumber of the UFBs are generated.

Conversely, when the pressure of the liquid increases to be higher thannormal temperature, the dissolution properties of the gas are increased,and the generated UFBs are more likely to be liquefied. However, suchpressure is sufficiently higher than the atmospheric pressure.Additionally, since the once generated UFBs have a high internalpressure and large gas-liquid interface energy even when the pressure ofthe liquid increases, it is highly unlikely that there is exerted asufficiently high pressure to break such a gas-liquid interface. Inother words, the once generated UFBs do not disappear easily as long asthe liquid is stored at normal temperature and normal pressure.

In this embodiment, the second UFBs 11B described with FIGS. 8A to 8Cand the fourth UFBs 11D described with FIGS. 10A to 10C can be describedas UFBs that are generated by utilizing such pressure dissolutionproperties of gas.

Those first to fourth UFBs generated by different causes are describedindividually above; however, the above-described generation causes occursimultaneously with the event of the film boiling. Thus, at least twotypes of the first to the fourth UFBs may be generated at the same time,and these generation causes may cooperate to generate the UFBs. Itshould be noted that it is common for all the generation causes to beinduced by the volume change of the film boiling bubble generated by thefilm boiling phenomenon. In this specification, the method of generatingthe UFBs by utilizing the film boiling caused by the rapid heating asdescribed above is referred to as a thermal-ultrafine bubble (T-UFB)generating method. Additionally, the UFBs generated by the T-UFBgenerating method are referred to as T-UFBs, and the liquid containingthe T-UFBs generated by the T-UFB generating method is referred to as aT-UFB-containing liquid.

Almost all the air bubbles generated by the T-UFB generating method are1.0 μm or less, and milli-bubbles and microbubbles are unlikely to begenerated. That is, the T-UFB generating method allows dominant andefficient generation of the UFBs. Additionally, the T-UFBs generated bythe T-UFB generating method have larger gas-liquid interface energy thanthat of the UFBs generated by a conventional method, and the T-UFBs donot disappear easily as long as being stored at normal temperature andnormal pressure. Moreover, even if new T-UFBs are generated by new filmboiling, it is possible to prevent disappearance of the alreadygenerated T-UFBs due to the impact from the new generation. That is, itcan be said that the number and the concentration of the T-UFBscontained in the T-UFB-containing liquid have the hysteresis propertiesdepending on the number of times the film boiling is made in theT-UFB-containing liquid. In other words, it is possible to adjust theconcentration of the T-UFBs contained in the T-UFB-containing liquid bycontrolling the number of the heating elements provided in the T-UFBgenerating unit 300 and the number of the voltage pulse application tothe heating elements.

Reference to FIG. 1 is made again. Once the T-UFB-containing liquid Wwith a desired UFB concentration is generated in the T-UFB generatingunit 300, the UFB-containing liquid W is supplied to the post-processingunit 400.

FIGS. 11A to 11C are diagrams illustrating configuration examples of thepost-processing unit 400 of this embodiment. The post-processing unit400 of this embodiment removes impurities in the UFB-containing liquid Win stages in the order from inorganic ions, organic substances, andinsoluble solid substances.

FIG. 11A illustrates a first post-processing mechanism 410 that removesthe inorganic ions. The first post-processing mechanism 410 includes anexchange container 411, cation exchange resins 412, a liquidintroduction passage 413, a collecting pipe 414, and a liquid dischargepassage 415. The exchange container 411 stores the cation exchangeresins 412. The UFB-containing liquid W generated by the T-UFBgenerating unit 300 is injected to the exchange container 411 throughthe liquid introduction passage 413 and absorbed into the cationexchange resins 412 such that the cations as the impurities are removed.Such impurities include metal materials peeled off from the elementsubstrate 12 of the T-UFB generating unit 300, such as SiO₂, SiN, SiC,Ta, Al₂O₃, Ta₂O₅, and Ir.

The cation exchange resins 412 are synthetic resins in which afunctional group (ion exchange group) is introduced in a high polymermatrix having a three-dimensional network, and the appearance of thesynthetic resins are spherical particles of around 0.4 to 0.7 mm. Ageneral high polymer matrix is the styrene-divinylbenzene copolymer, andthe functional group may be that of methacrylic acid series and acrylicacid series, for example. However, the above material is an example. Aslong as the material can remove desired inorganic ions effectively, theabove material can be changed to various materials. The UFB-containingliquid W absorbed in the cation exchange resins 412 to remove theinorganic ions is collected by the collecting pipe 414 and transferredto the next step through the liquid discharge passage 415. In thisprocess in the present embodiment, not all the inorganic ions containedin the UFB-containing liquid W supplied from the liquid introductionpassage 413 need to be removed as long as at least a part of theinorganic ions are removed.

FIG. 11B illustrates a second post-processing mechanism 420 that removesthe organic substances. The second post-processing mechanism 420includes a storage container 421, a filtration filter 422, a vacuum pump423, a valve 424, a liquid introduction passage 425, a liquid dischargepassage 426, and an air suction passage 427. Inside of the storagecontainer 421 is divided into upper and lower two regions by thefiltration filter 422. The liquid introduction passage 425 is connectedto the upper region of the upper and lower two regions, and the airsuction passage 427 and the liquid discharge passage 426 are connectedto the lower region thereof. Once the vacuum pump 423 is driven with thevalve 424 closed, the air in the storage container 421 is dischargedthrough the air suction passage 427 to make the pressure inside thestorage container 421 negative pressure, and the UFB-containing liquid Wis thereafter introduced from the liquid introduction passage 425. Then,the UFB-containing liquid W from which the impurities are removed by thefiltration filter 422 is reserved into the storage container 421.

The impurities removed by the filtration filter 422 include organicmaterials that may be mixed at a tube or each unit, such as organiccompounds including silicon, siloxane, and epoxy, for example. A filterfilm usable for the filtration filter 422 includes a filter of asub-μm-mesh (a filter of 1 μm or smaller in mesh diameter) that canremove bacteria, and a filter of a nm-mesh that can remove virus. Thefiltration filter having such a fine opening diameter may remove airbubbles larger than the opening diameter of the filter. Particularly,there may be the case where the filter is clogged by the fine airbubbles adsorbed to the openings (mesh) of the filter, which mayslowdown the filtering speed. However, as described above, most of theair bubbles generated by the T-UFB generating method described in thepresent embodiment of the invention are in the size of 1 μm or smallerin diameter, and milli-bubbles and microbubbles are not likely to begenerated. That is, since the probability of generating milli-bubblesand microbubbles is extremely low, it is possible to suppress theslowdown in the filtering speed due to the adsorption of the air bubblesto the filter. For this reason, it is favorable to apply the filtrationfilter 422 provided with the filter of 1 μm or smaller in mesh diameterto the system having the T-UFB generating method.

Examples of the filtration applicable to this embodiment may be aso-called dead-end filtration and cross-flow filtration. In the dead-endfiltration, the direction of the flow of the supplied liquid and thedirection of the flow of the filtration liquid passing through thefilter openings are the same, and specifically, the directions of theflows are made along with each other. In contrast, in the cross-flowfiltration, the supplied liquid flows in a direction along a filtersurface, and specifically, the direction of the flow of the suppliedliquid and the direction of the flow of the filtration liquid passingthrough the filter openings are crossed with each other. It ispreferable to apply the cross-flow filtration to suppress the adsorptionof the air bubbles to the filter openings.

After a certain amount of the UFB-containing liquid W is reserved in thestorage container 421, the vacuum pump 423 is stopped and the valve 424is opened to transfer the T-UFB-containing liquid in the storagecontainer 421 to the next step through the liquid discharge passage 426.Although the vacuum filtration method is employed as the method ofremoving the organic impurities herein, a gravity filtration method anda pressurized filtration can also be employed as the filtration methodusing a filter, for example.

FIG. 11C illustrates a third post-processing mechanism 430 that removesthe insoluble solid substances. The third post-processing mechanism 430includes a precipitation container 431, a liquid introduction passage432, a valve 433, and a liquid discharge passage 434.

First, a predetermined amount of the UFB-containing liquid W is reservedinto the precipitation container 431 through the liquid introductionpassage 432 with the valve 433 closed, and leaving it for a while.Meanwhile, the solid substances in the UFB-containing liquid W areprecipitated onto the bottom of the precipitation container 431 bygravity. Among the bubbles in the UFB-containing liquid, relativelylarge bubbles such as microbubbles are raised to the liquid surface bythe buoyancy and also removed from the UFB-containing liquid. After alapse of sufficient time, the valve 433 is opened, and theUFB-containing liquid W from which the solid substances and largebubbles are removed is transferred to the collecting unit 500 throughthe liquid discharge passage 434. The example of applying the threepost-processing mechanisms in sequence is shown in this embodiment;however, it is not limited thereto, and the order of the threepost-processing mechanisms may be changed, or at least one neededpost-processing mechanism may be employed.

Reference to FIG. 1 is made again. The T-UFB-containing liquid W fromwhich the impurities are removed by the post-processing unit 400 may bedirectly transferred to the collecting unit 500 or may be put back tothe dissolving unit 200 again to implement a circulation system. In thelatter case, the gas dissolution concentration of the T-UFB-containingliquid W that is decreased due to the generation of the T-UFBs can berisen. It is preferable that the gas dissolution concentration iscompensated to the saturated state again by the dissolving unit 200. Ifnew T-UFBs are generated by the T-UFB generating unit 300 after thecompensation, it is possible to further increase the concentration ofthe UFBs contained in the T-UFB-containing liquid with theabove-described properties. That is, it is possible to increase theconcentration of the contained UFBs by the number of circulationsthrough the dissolving unit 200, the T-UFB generating unit 300, and thepost-processing unit 400, and it is possible to transfer theUFB-containing liquid W to the collecting unit 500 after a predeterminedconcentration of the contained UFBs is obtained. This embodiment shows aform in which the UFB-containing liquid processed by the post-processingunit 400 is put back to the dissolving unit 200 and circulated; however,it is not limited thereto, and the UFB-containing liquid after passingthrough the T-UFB generating unit may be put back again to thedissolving unit 200 before being supplied to the post-processing unit400 such that the post-processing is performed by the post-processingunit 400 after the T-UFB concentration is increased through multipletimes of circulation, for example.

The collecting unit 500 collects and preserves the UFB-containing liquidW transferred from the post-processing unit 400. The T-UFB-containingliquid collected by the collecting unit 500 is a UFB-containing liquidwith high purity from which various impurities are removed.

In the collecting unit 500, the UFB-containing liquid W may beclassified by the size of the T-UFBs by performing some stages offiltration processing. Since it is expected that the temperature of theT-UFB-containing liquid W obtained by the T-UFB method is higher thannormal temperature, the collecting unit 500 may be provided with acooling unit. The cooling unit may be provided to a part of thepost-processing unit 400.

The schematic description of the UFB generating apparatus 1 is givenabove; however, it is needless to say that the illustrated multipleunits can be changed, and not all of them need to be prepared. Dependingon the type of the liquid W and the gas G to be used and the intendeduse of the T-UFB-containing liquid to be generated, a part of theabove-described units may be omitted, or another unit other than theabove-described units may be added.

For example, when the gas to be contained by the UFBs is the atmosphericair, the degassing unit as the pre-processing unit 100 and thedissolving unit 200 can be omitted. On the other hand, when multiplekinds of gases are desired to be contained by the UFBs, anotherdissolving unit 200 may be added.

The units for removing the impurities as described in FIGS. 11A to 11Cmay be provided upstream of the T-UFB generating unit 300 or may beprovided both upstream and downstream thereof. When the liquid to besupplied to the UFB generating apparatus is tap water, rain water,contaminated water, or the like, there may be included organic andinorganic impurities in the liquid. If such a liquid W including theimpurities is supplied to the T-UFB generating unit 300, there is a riskof deteriorating the heating element 10 and inducing the salting-outphenomenon. With the mechanisms as illustrated in FIGS. 11A to 11Cprovided upstream of the T-UFB generating unit 300, it is possible toremove the above-described impurities previously.

<<Specific Example of T-UFB Generating Method>>

Next, specific layouts of the heating element 10 and the flow passage inthe T-UFB generating unit 300 for efficient generation of the ultrafinebubbles are described with some embodiments.

Example 1

FIGS. 12A to 12C are plane diagrams of the heating element 10 ofExample 1. As illustrated in FIG. 12A, in this example, multiple surfacereforming regions 20 are formed at regular intervals vertically andhorizontally on the heating element 10. The surface reforming region 20is a region on the surface of the heating element 10 in which thesurface is processed to be rough. Such a region can be formed byirradiating the surface of the heating element 10 with a laser beam. Inthis example, uneven portions with a height difference of about 0.5 μmare formed in the surface reforming region 20.

On the surface of the heating element 10, the film boiling bubble 13 islikely to be generated earlier in the surface reforming region 20 with arelatively high surface roughness than in another region or anon-surface reforming region. That is, if multiple surface reformingregions 20 are arranged on one heating element 10 like this example, itis possible to generate the film boiling bubbles 13 in multiple parts ofthe heating element 10 simultaneously by one application of a voltagepulse to the heating element 10 (see FIG. 12B).

In this process, in each of the surface reforming regions 20, the firstto fourth UFBs 11A to 11D are generated by the thermal action and thepressure action described above exerted by the generation and thedisappearance of the film boiling bubble 13. In addition, there areformed portions between the adjacent surface reforming regions 20 inwhich the thermal action and the pressure action are mutually emphasizedand thus the gas-dissolved liquid 3 can exceed the thermal dissolutionlimit and the pressure dissolution limit more easily.

FIG. 12C illustrates the state where the shock waves made by thedisappearance of the film boiling bubble 13 are propagated in theadjacent two surface reforming regions 20. The shock waves ripple fromthe corresponding bubble disappearance points in the surface reformingregions 20 and interfere with each other. Thus, a pressurizedinterference point 18A in which the high pressure surfaces 17A meet andpressurization force is amplified and a depressurized interference point18B in which the low pressure surfaces 17B meet and depressurizationforce is amplified. This allows the gas-dissolved liquid 3 to exceedmore easily the pressure dissolution limit in the depressurizedinterference point 18B. As a result, a greater number of the fourth UFBs11D are generated than the case of one disappearance point.

That is, according to this example, the T-UFBs can be generated by thethermal action and the pressure action in each surface reforming region20, and also the T-UFBs can be generated by the interference between thethermal action and the pressure action generated between the multiplesurface reforming regions 20. Therefore, according to this example, itis possible to further improve the efficiency of the generation of theT-UFBs by forming the multiple surface reforming regions 20 on oneheating element 10. According to the studies by the inventors, it isconfirmed that the film boiling bubbles 13 can be generatedsimultaneously in the multiple surface reforming regions 20 as describedabove in a case where a surface roughness Ra of the surface reformingregions 20 is double or more a surface roughness of the non-surfacereforming regions.

Example 2

FIGS. 13A and 13B are plane diagrams of the heating element 10 ofExample 2. As illustrated in FIG. 13A, in this example, multiple heatingelements 10 are arrayed at distances d of 5 μm or less vertically andhorizontally on one element substrate 12. Each of the heating elements10 can be driven and controlled individually, and the adjacent multipleheating elements 10 can be driven simultaneously and also can be drivenin different timings.

FIG. 13B illustrates the state of growth of the film boiling bubbles 13generated on the heating elements 10. The upper section of the diagramherein shows the case where the adjacent heating elements 10 are drivenin different timings. In this case, the film boiling bubble 13 isgenerated and disappears in association with one heating element 10, andthe above-described first to fourth UFBs 11A to 11D are generated.

On the other hand, the lower section of the diagram shows the case wherethe adjacent heating elements 10 are driven in the same timing. In thiscase, the film boiling bubbles 13 generated in each of the heatingelements 10 become integrated during the growing process and form asingle connected film boiling bubble 13A. According to the studies bythe inventors, it is confirmed that such a connected film boiling bubble13A can be properly generated in a case where the distances d in thelayout of the multiple heating elements 10 are set to 5 μm or less.

In the generation and disappearance processes of the connected filmboiling bubble 13A, it is possible to generate UFBs of a different sizeor generate the first to fourth UFBs 11A to 11D of a different balancefrom the UFBs generated in the generation and disappearance processes ofthe single boiling bubble 13 that is not connected with the other. Thatis, in a case where the layout of the multiple heating elements is madewith distances of 5 μm or less in advance like this example, it ispossible to adjust the size and the balance of the UFBs to be generatedin various ways depending on needs by selectively changing the positionsand the number of the heating elements 10 to be driven simultaneously.

Example 3

FIGS. 14A to 14C are plane diagrams illustrating the states of the filmboiling bubble 13 and the flow passage in which the film boiling bubble13 is generated in Example 3. There is shown the state where the liquidW is stored in the flow passage restricted by two opposing flow passagewalls 30, the film boiling bubble 13 generated in the flow passage growsalong the flow passage walls 30 on two sides, and the film boilingbubble 13 shrinks eventually. Although FIGS. 14A to 14C are diagrams asviewed from the opposing side of the element substrate 12 and theheating element 10, the element substrate 12 and the heating element 10are not illustrated therein.

When the film boiling bubble 13 grows along the flow passage walls 30,reaches the maximum volume, and then shrinks, the shrinking speed of theportions in contact with the flow passage walls 30 are affected by theflow passage resistance from the flow passage walls 30 and therebybecomes slower than the shrinking speed of the middle portion away fromthe flow passage walls 30 (see FIG. 14A). Consequently, a collision inthe film boiling bubble 13 occurs in the middle portion first, and thenthe shock waves generated from the point P1 made by the collision arepropagated first in the liquid W (see FIG. 14B). Thereafter, suchcollisions occur in multiple portions sequentially in upward anddownward directions from the point P1, and the film boiling bubble 13eventually disappears in two portions (a point P2 and a point P3)vertically away from each other (see FIG. 14C).

As described above, if the flow passage walls 30 restricting thedirections of growth of the film boiling bubble 13 are provided inadvance, it is possible to form the disappearing film boiling bubble 13in a projection shape, and this makes it possible to extend the requiredtime from the start (FIG. 14B) to the end (FIG. 14C) of the collision.As a result, it is possible to generate further more small UFBs than thecase where no flow passage walls 30 are provided and the film boilingbubble 13 disappears in a short time by only the negative pressurethereof.

Example 4

FIGS. 15A and 15B are plane diagrams illustrating the states of the filmboiling bubble 13 and the flow passage in which the film boiling bubble13 is generated in Example 4. In the flow passage of this example, fluidresistance elements 40 as resistances to fluid are further added in theflow passage in Example 3. The fluid resistance elements 40 are providedto be positioned in substantially the middle of the two flow passagewalls 30 and on two sides of the heating element 10 (which is notillustrated in FIGS. 15A and 15B).

As already described in Example 3, with the flow passage walls 30restricting the shape of the film boiling bubble 13 provided, it ispossible to make the shrinking speed of the portions close to the flowpassage walls 30 slower in the film boiling bubble 13. In addition tothis, in a case where the fluid resistance elements 40 are provided inthe portion away from the flow passage walls 30 like this example, it ispossible to make the shrinking speed of the portion away from the flowpassage walls 30 slower as well, and eventually it is possible to makethe shrinking speed of the entire film boiling bubble 13 slower.Moreover, it is possible to adjust the shape of the shrinking filmboiling bubble 13 and the required time from the collision to thedisappearance by changing the distance between the opposing flow passagewalls 30 and the number and the positions of the fluid resistanceelements 40. FIGS. 15A and 15B show an example in which the shrinkingspeed of the portions close to and the portion away from the flowpassage walls 30 of the film boiling bubble 13, or the shrinking speedin a direction orthogonal to the flow passage walls 30 is madesubstantially equal to make adjustment so that the collisions and thedisappearance of the film boiling bubble 13 occur simultaneously on asurface R.

In this case, since the collisions on the surface R can occursubstantially at the same time, Peak-Peak of the shock waves propagatedin the liquid W is also increased. As a result, it is possible togenerate larger T-UFBs with larger gas-liquid interface energy.

On the other hand, FIGS. 16A to 16C illustrate an example in whichfurther more fluid resistance elements 40 are arranged than that ofFIGS. 15A and 15B. In this case, the shrinking speed of the portion awayfrom the flow passage walls 30 can be further slower than the shrinkingspeed of the portions close to the flow passage walls 30. This causesthe disappearing film boiling bubble 13 to be formed in a recess shape(see FIG. 16A), and the first collisions occur in upper and lower twoends close to the flow passage walls 30. Thereafter, such collisionsoccur sequentially in multiple portions from the top and the bottom tothe middle (see FIG. 16B), and the film boiling bubble 13 disappears inthe central point P eventually (see FIG. 16C).

In this example, the required time for the shrinkage of the entire filmboiling bubble 13 and the collisions is longer than that of Example 3.This makes it possible to suppress the collision energy in thedisappearance and distribute the stress to the heating element 10. As aresult, it is possible to extend the lifetime of the heating element 10,and thus the running cost of the T-UFB generating unit 300 can bereduced.

<<Liquid and Gas Usable For T-UFB-Containing Liquid>>

Now, the liquid W usable for generating the T-UFB-containing liquid isdescribed. The liquid W usable in this embodiment is, for example, purewater, ion exchange water, distilled water, bioactive water, magneticactive water, lotion, tap water, sea water, river water, clean andsewage water, lake water, underground water, rain water, and so on. Amixed liquid containing the above liquid and the like is also usable. Amixed solvent containing water and soluble organic solvent can be alsoused. The soluble organic solvent to be used by being mixed with wateris not particularly limited; however, the followings can be a specificexample thereof. An alkyl alcohol group of the carbon number of 1 to 4including methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropylalcohol, n-butyl alcohol, sec-butyl alcohol, and tert-butyl alcohol. Anamide group including N-methyl-2-pyrrolidone, 2-pyrrolidone,1,3-dimethyl-2-imidazolidinone, N,N-dimethylformamide, andN,N-dimethylacetamide. A keton group or a ketoalcohol group includingacetone and diacetone alcohol. A cyclic ether group includingtetrahydrofuran and dioxane. A glycol group including ethylene glycol,1,2-propylene glycol, 1,3-propylene glycol, 1,2-butanediol,1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,2-hexanediol,1,6-hexanediol, 3-methyl-1,5-pentanediol, diethylene glycol, triethyleneglycol, and thiodiglycol. A group of lower alkyl ether of polyhydricalcohol including ethylene glycol monomethyl ether, ethylene glycolmonoethyl ether, ethylene glycol monobutyl ether, diethylene glycolmonomethyl ether, diethylene glycol monoethyl ether, diethylene glycolmonobutyl ether, triethylene glycol monomethyl ether, triethylene glycolmonoethyl ether, and triethylene glycol monobutyl ether. A polyalkyleneglycol group including polyethylene glycol and polypropylene glycol. Atriol group including glycerin, 1,2,6-hexanetriol, andtrimethylolpropane. These soluble organic solvents can be usedindividually, or two or more of them can be used together.

A gas component that can be introduced into the dissolving unit 200 is,for example, hydrogen, helium, oxygen, nitrogen, methane, fluorine,neon, carbon dioxide, ozone, argon, chlorine, ethane, propane, air, andso on. The gas component may be a mixed gas containing some of theabove. Additionally, it is not necessary for the dissolving unit 200 todissolve a substance in a gas state, and the dissolving unit 200 mayfuse a liquid or a solid containing desired components into the liquidW. The dissolution in this case may be spontaneous dissolution,dissolution caused by pressure application, or dissolution caused byhydration, ionization, and chemical reaction due to electrolyticdissociation.

<<Effects of T-UFB Generating Method>>

Next, the characteristics and the effects of the above-described T-UFBgenerating method are described by comparing with a conventional UFBgenerating method. For example, in a conventional air bubble generatingapparatus as represented by the Venturi method, a mechanicaldepressurizing structure such as a depressurizing nozzle is provided ina part of a flow passage. A liquid flows at a predetermined pressure topass through the depressurizing structure, and air bubbles of varioussizes are generated in a downstream region of the depressurizingstructure.

In this case, among the generated air bubbles, since the relativelylarge bubbles such as milli-bubbles and microbubbles are affected by thebuoyancy, such bubbles rise to the liquid surface and disappear. Eventhe UFBs that are not affected by the buoyancy may also disappear withthe milli-bubbles and microbubbles since the gas-liquid interface energyof the UFBs is not very large. Additionally, even if the above-describeddepressurizing structures are arranged in series, and the same liquidflows through the depressurizing structures repeatedly, it is impossibleto store for a long time the UFBs of the number corresponding to thenumber of repetitions. In other words, it has been difficult for theUFB-containing liquid generated by the conventional UFB generatingmethod to maintain the concentration of the contained UFBs at apredetermined value for a long time.

In contrast, in the T-UFB generating method of this embodiment utilizingthe film boiling, a rapid temperature change from normal temperature toabout 300° C. and a rapid pressure change from normal pressure to arounda several megapascal occur locally in a part extremely close to theheating element. The heating element is a rectangular shape having oneside of around several tens to hundreds of μm. It is around 1/10 to1/1000 of the size of a conventional UFB generating unit. Additionally,with the gas-dissolved liquid within the extremely thin film region ofthe film boiling bubble surface exceeding the thermal dissolution limitor the pressure dissolution limit instantaneously (in an extremely shorttime under microseconds), the phase transition occurs and thegas-dissolved liquid is precipitated as the UFBs. In this case, therelatively large bubbles such as milli-bubbles and microbubbles arehardly generated, and the liquid contains the UFBs of about 100 nm indiameter with extremely high purity. Moreover, since the T-UFBsgenerated in this way have sufficiently large gas-liquid interfaceenergy, the T-UFBs are not broken easily under the normal environmentand can be stored for a long time.

Particularly, the present invention using the film boiling phenomenonthat enables local formation of a gas interface in the liquid can forman interface in a part of the liquid close to the heating elementwithout affecting the entire liquid region, and a region on which thethermal and pressure actions performed can be extremely local. As aresult, it is possible to stably generate desired UFBs. With furthermore conditions for generating the UFBs applied to the generation liquidthrough the liquid circulation, it is possible to additionally generatenew UFBs with small effects on the already-made UFBs. As a result, it ispossible to produce a UFB liquid of a desired size and concentrationrelatively easily.

Moreover, since the T-UFB generating method has the above-describedhysteresis properties, it is possible to increase the concentration to adesired concentration while keeping the high purity. In other words,according to the T-UFB generating method, it is possible to efficientlygenerate a long-time storable UFB-containing liquid with high purity andhigh concentration.

<<Specific Usage of T-UFB-Containing Liquid>>

In general, applications of the ultrafine bubble-containing liquids aredistinguished by the type of the containing gas. Any type of gas canmake the UFBs as long as an amount of around PPM to BPM of the gas canbe dissolved in the liquid. For example, the ultrafine bubble-containingliquids can be applied to the following applications.

-   -   A UFB-containing liquid containing air can be preferably applied        to cleansing in the industrial, agricultural and fishery, and        medical scenes and the like, and to cultivation of plants and        agricultural and fishery products.    -   A UFB-containing liquid containing ozone can be preferably        applied to not only cleansing application in the industrial,        agricultural and fishery, and medical scenes and the like, but        to also applications intended to disinfection, sterilization,        and decontamination, and environmental cleanup of drainage and        contaminated soil, for example.    -   A UFB-containing liquid containing nitrogen can be preferably        applied to not only cleansing application in the industrial,        agricultural and fishery, and medical scenes and the like, but        to also applications intended to disinfection, sterilization,        and decontamination, and environmental cleanup of drainage and        contaminated soil, for example.    -   A UFB-containing liquid containing oxygen can be preferably        applied to cleansing application in the industrial, agricultural        and fishery, and medical scenes and the like, and to cultivation        of plants and agricultural and fishery products.    -   A UFB-containing liquid containing carbon dioxide can be        preferably applied to not only cleansing application in the        industrial, agricultural and fishery, and medical scenes and the        like, but to also applications intended to disinfection,        sterilization, and decontamination, for example.    -   A UFB-containing liquid containing perfluorocarbons as a medical        gas can be preferably applied to ultrasonic diagnosis and        treatment. As described above, the UFB-containing liquids can        exert the effects in various fields of medical, chemical,        dental, food, industrial, agricultural and fishery, and so on.

In each of the applications, the purity and the concentration of theUFBs contained in the UFB-containing liquid are important for quicklyand reliably exert the effect of the UFB-containing liquid. In otherwords, unprecedented effects can be expected in various fields byutilizing the T-UFB generating method of this embodiment that enablesgeneration of the UFB-containing liquid with high purity and desiredconcentration. Here is below a list of the applications in which theT-UFB generating method and the T-UFB-containing liquid are expected tobe preferably applicable.

(A) Liquid Purification Application

-   -   With the T-UFB generating unit provided to a water clarification        unit, enhancement of an effect of water clarification and an        effect of purification of PH adjustment liquid is expected. The        T-UFB generating unit may also be provided to a carbonated water        server.    -   With the T-UFB generating unit provided to a humidifier, aroma        diffuser, coffee maker, and the like, enhancement of a        humidifying effect, a deodorant effect, and a scent spreading        effect in a room is expected.    -   If the UFB-containing liquid in which an ozone gas is dissolved        by the dissolving unit is generated and is used for dental        treatment, burn treatment, and wound treatment using an        endoscope, enhancement of a medical cleansing effect and an        antiseptic effect is expected.    -   With the T-UFB generating unit provided to a water storage tank        of a condominium, enhancement of a water clarification effect        and chlorine removing effect of drinking water to be stored for        a long time is expected.    -   If the T-UFB-containing liquid containing ozone or carbon        dioxide is used for brewing process of Japanese sake, shochu,        wine, and so on in which the high-temperature pasteurization        processing cannot be performed, more efficient pasteurization        processing than that with the conventional liquid is expected.    -   If the UFB-containing liquid is mixed into the ingredient in a        production process of the foods for specified health use and the        foods with functional claims, the pasteurization processing is        possible, and thus it is possible to provide safe and functional        foods without a loss of flavor.    -   With the T-UFB generating unit provided to a supplying route of        sea water and fresh water for cultivation in a cultivation place        of fishery products such as fish and pearl, prompting of        spawning and growing of the fishery products is expected.    -   With the T-UFB generating unit provided in a purification        process of water for food preservation, enhancement of the        preservation state of the food is expected.    -   With the T-UFB generating unit provided in a bleaching unit for        bleaching pool water or underground water, a higher bleaching        effect is expected.    -   With the T-UFB-containing liquid used for repairing a crack of a        concrete member, enhancement of the effect of crack repairment        is expected.    -   With the T-UFBs contained in liquid fuel for a machine using        liquid fuel (such as automobile, vessel, and airplane),        enhancement of energy efficiency of the fuel is expected.        (B) Cleansing Application

Recently, the UFB-containing liquids have been receiving attention ascleansing water for removing soils and the like attached to clothing. Ifthe T-UFB generating unit described in the above embodiment is providedto a washing machine, and the UFB-containing liquid with higher purityand better permeability than the conventional liquid is supplied to thewashing tub, further enhancement of detergency is expected.

-   -   With the T-UFB generating unit provided to a bath shower and a        bedpan washer, not only a cleansing effect on all kinds of        animals including human body but also an effect of prompting        contamination removal of a water stain and a mold on a bathroom        and a bedpan are expected.    -   With the T-UFB generating unit provided to a window washer for        automobiles, a high-pressure washer for cleansing wall members        and the like, a car washer, a dishwasher, a food washer, and the        like, further enhancement of the cleansing effects thereof is        expected.    -   With the T-UFB-containing liquid used for cleansing and        maintenance of parts produced in a factory including a burring        step after pressing, enhancement of the cleansing effect is        expected.    -   In production of semiconductor elements, if the T-UFB-containing        liquid is used as polishing water for a wafer, enhancement of        the polishing effect is expected. Additionally, if the        T-UFB-containing liquid is used in a resist removal step,        prompting of peeling of resist that is not peeled off easily is        enhanced.    -   With the T-UFB generating unit is provided to machines for        cleansing and decontaminating medical machines such as a medical        robot, a dental treatment unit, an organ preservation container,        and the like, enhancement of the cleansing effect and the        decontamination effect of the machines is expected. The T-UFB        generating unit is also applicable to treatment of animals.        (C) Pharmaceutical Application    -   If the T-UFB-containing liquid is contained in cosmetics and the        like, permeation into subcutaneous cells is prompted, and        additives that give bad effects to skin such as preservative and        surfactant can be reduced greatly. As a result, it is possible        to provide safer and more functional cosmetics.    -   If a high concentration nanobubble preparation containing the        T-UFBs is used for contrasts for medical examination apparatuses        such as a CT and an MRI, reflected light of X-rays and        ultrasonic waves can be efficiently used. This makes it possible        to capture a more detailed image that is usable for initial        diagnosis of a cancer and the like.    -   If a high concentration nanobubble water containing the T-UFBs        is used for a ultrasonic wave treatment machine called        high-intensity focused ultrasound (HIFU), the irradiation power        of ultrasonic waves can be reduced, and thus the treatment can        be made more non-invasive. Particularly, it is possible to        reduce the damage to normal tissues.    -   It is possible to create a nanobubble preparation by using high        concentration nanobubbles containing the T-UFBs as a source,        modifying a phospholipid forming a liposome in a negative        electric charge region around the air bubble, and applying        various medical substances (such as DNA and RNA) through the        phospholipid.    -   If a drug containing high concentration nanobubble water made by        the T-UFB generation is transferred into a dental canal for        regenerative treatment of pulp and dentine, the drug enters        deeply a dentinal tubule by the permeation effect of the        nanobubble water, and the decontamination effect is prompted.        This makes it possible to treat the infected root canal of the        pulp safely in a short time.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2019-035814 filed Feb. 28, 2019, which is hereby incorporated byreference wherein in its entirety.

What is claimed is:
 1. An ultrafine bubble generating method comprising:generating ultrafine bubbles smaller than 1.0 μm in diameter by causinga heating element provided in a liquid to generate heat, making filmboiling on an interface between the liquid and the heating element, andgenerating a film boiling bubble; and collecting the liquid containingthe ultrafine bubbles generated in the generating step.
 2. The ultrafinebubble generating method according to claim 1, wherein the ultrafinebubbles include at least one of first ultrafine bubbles generated near asurface of the film boiling bubble when the film boiling bubble isexpanded, second ultrafine bubbles generated near the surface of thefilm boiling bubble when the film boiling bubble shrinks, thirdultrafine bubbles generated near a surface of the heating element whenthe film boiling bubble shrinks, and fourth ultrafine bubbles generatedin a region in which a shock wave that is made when the film boilingbubble disappears is propagated.
 3. The ultrafine bubble generatingmethod according to claim 1, wherein the ultrafine bubbles are generatedin a case where a gas dissolved in the liquid exceeds a thermaldissolution limit, and undergoes a phase transition.
 4. The ultrafinebubble generating method according to claim 1, wherein the ultrafinebubbles are generated in a case where a gas dissolved in the liquidexceeds a pressure dissolution limit, and undergoes a phase transition.5. An ultrafine bubble generating method, comprising: dissolving apredetermined gas component into a liquid; generating ultrafine bubblessmaller than 1.0 μm in diameter containing the predetermined gascomponent by causing a heating element provided in the liquid in whichthe gas component is dissolved in the dissolving step to generate heat,making film boiling on an interface between the liquid and the heatingelement, and generating a film boiling bubble; and collecting the liquidcontaining the ultrafine bubbles generated in the generating step.